These days, every time you purchase something, you get a detailed receipt. With a receipt, you know exactly where your money is going and just how smart—or ridiculous—your spending decisions have been.

Not so with mutual funds. As a mutual fund investor, you'll never write a check to a mutual fund for its services. That means you'll never know exactly where your money is going unless you're very, very vigilant. Any service charges for mutual funds come right off the top of your investment or straight out of your returns. Because costs are deducted this way, many investors aren't even aware of how much they're paying for their mutual funds.

Mutual fund fees can be broken down into two main categories: one-time fees and ongoing annual expenses. Not all funds charge one-time fees, but all funds charge ongoing annual fees of some sort. Return figures that you see for mutual funds in newspapers, annual reports, and financial Web sites typically don't reflect one-time fees, but ongoing expenses are almost always deducted from return figures that you see in public sources.